Amyotrophic lateral sclerosis (ALS) is the most common motor neuron disease, and despite its initial description over 100 years ago by Jean-Martin Charcot, there remains no effective therapy for the ultimately fatal weakness and muscle atrophy typical of the disorder. Likewise, our ability to treat patients with frontotemporal dementia (FTD), the second most common type of dementia in individuals under the age of 65, is severely limited. Despite their clinical disparity, the majority of ALS and the most prevalent type of FTD share a common pathology marked by the deposition of the TAR DNA binding protein of 43 kDa (TDP-43). The identification of mutations within the gene encoding TDP-43 (TARDBP) and their association with familial ALS and FTD suggested that TDP-43 plays an integral role in disease pathogenesis. This proposal explores the cellular mechanisms responsible for mutant TDP43-induced neuronal loss, laying the foundation for the development of therapies that can prevent ALS, FTD, and other neurodegenerative conditions marked by TDP- 43 accumulation. In preliminary studies, we established a faithful neuronal model of TDP43-proteinopathies and verified that this system recapitulates essential features of disease in vitro. Furthermore, we demonstrated that a mutation in TARDBP associated with familial ALS is capable of causing neuronal toxicity through the mislocalization of TDP-43 from the nucleus, where it is normally concentrated, to the cytoplasm. An identical cytoplasmic redistribution of TDP-43 is characteristic of degenerating neurons from patients with ALS and FTD, confirming the significance of the phenomenon and directly implicating it in the pathogenesis of disease. Specific Aims 1 and 2 focus upon the potential mechanisms underlying the cytoplasmic redistribution of mutant TDP-43. Interestingly, mutations in TARDBP result invariably in symptoms of ALS with motor neuron degeneration, but only rarely in dementia with cortical neuron pathology, suggesting that motor neurons are selectively vulnerable to mutant TDP-43. In Specific Aim 3, I characterize the particular susceptibility of motor neurons to mutant TDP-43. The fundamental goal of the project is to define the pathways involved in mutant TDP43-mediated neurodegeneration, with the ultimate intention of devising therapies with the power to prevent or reverse disease progression. Because TDP-43 deposition is a fundamental property of spontaneous and familial ALS, as well as the most common pathologic subtype of FTD, investigations into TDP-43's contribution to disease pathogenesis will be critical if we are to eventually develop effective therapies. As principal investigator on the project, I have a strong background in neurology and neuroscience, and will have the support of a primary sponsor, a dedicated advisory committee, and two institutions (the Gladstone Institute and the University of California, San Francisco) in the pursuit of this goal. The research described in this application will lay the foundation for my career in the field of neurodegenerative disease. Moreover, with the receipt of this Award, I will gain the opportunity to acquire the skills, knowledge, mentoring, and experience to become a successful and independent physician-scientist.